The present disclosure relates to diodes using a nitride semiconductor, and more particularly relates to a diode which can be used as a power device for use in, e.g., a power supply circuit.
Nitride semiconductors represented by gallium nitride (GaN) are wide gap semiconductors, and for example, the band gaps of GaN and aluminum nitride (AlN) at room temperature are 3.4 eV, and 6.2 eV, respectively. The nitride semiconductors have a high breakdown field, and a higher saturated electron drift velocity than compound semiconductors, such as gallium arsenide (GaAs), and silicon (Si) semiconductors, etc. In an AlGaN/GaN heterostructure of aluminum gallium nitride (AlGaN) and GaN on plane (0001), 2-dimensional electron gas (2DEG) is generated at the heterointerface due to spontaneous polarization and piezoelectric polarization. The sheet carrier concentration of 2DEG is 1×1013 cm−2 or higher without impurity doping, and a diode and a hetero junction field effect transistor (HFET) both having high current density can be achieved by utilizing 2DEG. Thus, power devices using nitride semiconductors which are advantageous in increasing output power and breakdown voltage have been actively researched and developed. Examples of diodes used as power devices include Schottky diodes. Schottky diodes using an AlGaN/GaN heterostructure have been developed as GaN-based diodes. Since a Schottky diode using an AlGaN/GaN heterostructure uses, as a channel, 2DEG generated at the interface between an undoped AlGaN layer and an undoped GaN layer, such a Schottky diode has low resistance, and can be operated at large current.
Generally, while Schottky diodes advantageously have excellent switching performance, they disadvantageously have large reverse leakage current. In order to reduce the reverse leakage current of a Schottky diode, the following method has been suggested (see, for example, Japanese Patent Publication No. 2005-317843). Two types of metals having different Schottky barrier heights are used as materials of an anode electrode, and one of the metals having a lower Schottky barrier height is covered with the other metal having a higher Schottky barrier height. With such a configuration, when the diode is forwardly biased, current flows through the metal having a lower Schottky barrier height, and thus, a low threshold voltage can be maintained, resulting in a reduction in the conduction loss of the diode. When the diode is reversely biased, the diode can be turned off by the metal having a higher Schottky barrier height, and thus, there is an expectation that the reverse leakage current could be reduced.